1. Field of the Invention
The present invention relates in general to integrated circuit (IC) testers and in particular to a load circuit for providing an adjustable load current at a terminal of an IC device under test.
2. Description of Related Art
A typical IC tester includes a set of pin electronics circuits, one for each pin or terminal of an IC device under test (DUT). Each pin electronics circuit is capable of either transmitting a test signal to a DUT terminal or sampling an IC output signal produced at the DUT terminal to determine its logic state. Normally, when sampling a DUT output signal, the pin electronics circuit must provide, with reasonably high accuracy, a specified load current into or out of the IC terminal depending on whether the IC output signal is at a specified low or high logic level.
FIG. 1 illustrates a typical prior art load circuit 10 for providing adjustable high and low logic level load currents I.sub.OH and I.sub.OL at a DUT terminal 12. A unity gain operational amplifier 13 receives an input reference threshold voltage V.sub.th and supplies it to a bridge-connected switching diode quad 14 also coupled to DUT terminal 12. An adjustable current source 16 controlled by an input reference voltage V.sub.L supplies a current I.sub.OL into diode quad 14. Another adjustable current source 18 controlled by input voltage V.sub.H draws a current I.sub.OH from diode quad 14. When the DUT output signal at terminal 12 falls to a low logic level sufficiently below the threshold voltage V.sub.TH, diode quad 14 connects current source 16 to DUT terminal 12 instead of to amplifier 13 so that DUT terminal 12 receives the specified low logic level load current I.sub.OL. Conversely, when the DUT output signal at terminal 12 rises to a high logic level sufficiently above threshold voltage V.sub.TH, diode quad 14 connects current source 18 to DUT terminal 12 instead of to amplifier 13 so that the specified high logic level load current I.sub.OH is drawn from DUT terminal 12.
FIG. 2 illustrates a typical prior art current source 18 as has been used in load circuit 10 of FIG. 1. Current source 18 includes an operational amplifier 22 receiving the input control voltage V.sub.H and driving the base of a transistor 24. The emitter of transistor 24 drives the base of another transistor 26 having an emitter coupled to V.sub.H (common) through a switched resistor network 28 and to an inverting input of operational amplifier 22. The collectors of transistors 24 and 26 are interconnected and draw the output current I.sub.OH. Operational amplifier 22 supplies sufficient voltage to the base of transistor 24 so that the voltage V'.sub.H appearing across resistor network 28 is held substantially equal to V.sub.H. Thus the current through resistor network 28 is substantially equal to V.sub.H /R.sub.T where R.sub.T is the resistance on resistor network 28. Since the current into the base of transistor 24 is very small, the current source 18 output current I.sub.OH drawn by the collectors of transistors 24 and 26 is also substantially equal to V.sub.H /R.sub.T. The resistance of resistor network 28 is selected by input signal RANGE controlling the network switches. Current source 16 of FIG. 1 is analogous to circuit 18 but is constructed using PNP transistors instead of NPN transistors so that is supplies a positive current output.
The input control voltage V.sub.H is typically generated by a digital-to-analog (A/D) converter (not shown) which normally can vary the control voltage V.sub.H over only a relatively limited range, for example from 0 to +3V with a resolution of 3V/256 for an 8-bit A/D converter. Specified DUT load currents produced in response to that control-voltage can vary over a wide range, for example from +/-0.1 microamp to +/-100 milliamps. Since the transfer function of current source 18 relating output current I.sub.OH to input voltage V.sub.H is linear, we could obtain a wide output current range by using single small resistor at the emitter of transistor 26. However since the resolution with which the output current can be adjusted would be 100 milliamps/256 (about 400 microamps), then at the lower end of its range, the resolution with which the output load current could be adjusted would be unacceptably poor. By using the switched resistor network 28, rather than a single resistor, at the emitter of transistor 26, current source 18 provides several output current ranges, each having an acceptable resolution.
The adjustable current source 18 of FIG. 2 has a drawback in that resistor network 28 requires the use of discrete components (resistors and relay or FET switches) which cannot be included in an integrated circuit implementing the rest of current source 18. Since an integrated circuit tester requires a large number of load circuits, the many discrete component resistor networks needed add substantial cost and size to an IC tester. What is needed is programmable load circuit that can be implemented on a single integrated circuit without need of external discrete components and which can provide a wide range of output load currents with adequate resolution in response to a relatively narrow input control voltage range.